1. Field of the Invention
The present disclosure relates generally to internal combustion engines and more particularly to two-stroke engines.
2. Description of the Related Art
Internal-combustion engines, e.g., piston engines, fall into two main categories: two-stroke and four-stroke. In general, two-stroke, or two-cycle, engines are much less expensive to manufacture, use less moving parts, produce significantly more power for the same engine displacement, and weigh significantly less. These benefits arise primarily because the two-stroke engine, as compared to the four-stroke engine, generates a power stroke for every one revolution of the crankshaft, rather than every two revolutions of the crankshaft as in the four-stroke engine. The traditional two-stroke engine is a simple and robust design that: uses static cylinder ports rather than a dynamic valve train; lubricates the engine via an oil-laced fuel-air mixture traveling through the crankcase rather than having a separate wet or dry sump crankcase lubrication; and pumps the air-fuel-oil mixture into the cylinder intake using the crankcase pressure rather than ‘pulling’ it in via natural aspiration or pumping it in via a turbocharger or supercharger.
In a two-stroke engine, each downward stroke of the piston acts as a power stroke. The air-gas-oil mixture is pumped into a cylinder of the engine through an intake port or valve at a sufficient momentum and sufficiently high pressure to help discharge the burned gases from the cylinder through the exhaust port, via a process known as scavenging. A traditional two-stroke engine accomplishes this by pumping the intake charge into the intake using crankcase pressure. That is, the air-fuel-oil mixture is pushed into the lower-pressure crankcase through a valve, e.g., an open reed valve, during an upstroke of the piston, and the intake charge is then pumped out of the crankcase on the down stroke of a piston, when the reed valve is closed.
In order to provide lubrication to the moving parts in the engine, the air or air-fuel mixture is laced with lubricating oil. By adding oil to the air or air-fuel mixture, the crankcase is adequately lubricated. However, several detrimental effects arise from this practice. First, when gas is mixed in with the oil, the lubricating effects of the oil are reduced. Additionally, if the oil is improperly mixed with the gas or is improperly supplied to engine parts, then severe engine damage can arise. Thus a need arises to lubricate the engine without gasoline contamination.
A second detrimental effect of mixing fuel with oil is that oil residue remains in the air or air-fuel mixture as it is burned in the power stroke of the engine thereby producing significant amounts of air and/or water pollution, reducing engine power and fuel efficiency; and creating reliability problems and rough idling arising from an oil-fouled spark plug(s). Air pollution from two-stroke engines is exceptionally noticeable in highly populated developing countries because the engines are inexpensive, and the pollution laws rarely exist or are rarely enforced; a combination that encourages the use and application of two-stroke engines. In fact, in a survey conducted by the Bangladesh Road Transport Authority (BRTA), two-stroke petrol engines were found to be less fuel-efficient, and to emit about 30-100 times more unburned hydrocarbons than four-stroke engines. The inherent pollution from conventional two-stroke gas engines is recognized worldwide as one of the biggest current pollution problems and thus has spurred attempts to outlaw and restrict their use worldwide. Thus, a need arises to overcome the significant drawback of pollution caused by a two-stroke engine application and use.
If a two-stroke engine, utilizes a sealed oil-reserve crankcase, similar to that of a four-stroke engine, then it may not contaminate the air or air-fuel mixture with crankcase oil. However, neither does it utilize the natural pumping from the crankcase to pump the air or air-fuel mixture into the cylinder. Instead it may use a crankshaft-powered Roots type supercharger or an exhaust-powered turbocharger, which can add cost, weight, complexity, and possibly a boost lag. Thus a need arises for a two-stroke engine that both reduces oil pollution and uses crankcase pressure to pump the intake charge.
If an alternative two-stroke engine design utilizes the pumping action of the crankcase to force air or an air-fuel mixture to the combustion chamber but fails to use a barrier, then lubricating oil provided to the crankcase, even if by injector, still has the opportunity of entering the combustion chamber. Thus, a need still exists to provide a two-stroke engine design with substantially reduced oil contamination in the air or air-fuel mixture delivered to the combustion chamber mixture as opposed to reduced oil in the gas mixture on only fuel injected engines.